Renal Relevant Radiology: Use of Ultrasonography in Patients with AKI Sarah Faubel,* Nayana U. Patel,† Mark E. Lockhart,‡ and Melissa A. Cadnapaphornchai§ Summary As judged by the American College of Radiology Appropriateness Criteria, renal Doppler ultrasonography is the most appropriate imaging test in the evaluation of AKI and has the highest level of recommendation. Unfortunately, nephrologists are rarely specifically trained in ultrasonography technique and interpretation, and *Division of Internal Medicine, Nephrology, important clinical information obtained from renal ultrasonography may not be appreciated. In this review, the University of Colorado strengths and limitations of grayscale ultrasonography in the evaluation of patients with AKI will be discussed and Denver Veterans with attention to its use for (1) assessment of intrinsic causes of AKI, (2) distinguishing acute from chronic kidney Affairs Medical Center, 3 Denver, Colorado; diseases, and ( ) detection of obstruction. The use of Doppler imaging and the resistive index in patients with † AKI will be reviewed with attention to its use for (1) predicting the development of AKI, (2) predicting the Department of 3 Radiology and prognosis of AKI, and ( ) distinguishing prerenal azotemia from intrinsic AKI. Finally, pediatric considerations in §Department of Internal the use of ultrasonography in AKI will be reviewed. Medicine and Clin J Am Soc Nephrol 9: 382–394, 2014. doi: 10.2215/CJN.04840513 Pediatrics, Nephrology, University of Colorado Denver, Denver, Colorado; and Introduction structures on ultrasonography images. The renal cap- ‡Department of Renal ultrasonography is typically the most appro- sule consists of thin fibrous tissue, which is next to Radiology, University of priate and useful radiologic test in the evaluation of fat, and thus the kidney often appears to be surroun- Alabama at Birmingham, patients with AKI (1). In this report, we review the ded by a very bright rim on ultrasonography when Birmingham, Alabama interpretation of Doppler ultrasonography in adult there is a difference of acoustic impedance relative to and pediatric patients with AKI, with attention to the adjacent tissues. Solid organs, such as the liver Correspondence: its most appropriate uses, which include (1) the as- and spleen, have intermediate echogenicity, and the Dr. Sarah Faubel, sessment of intrinsic causes of AKI, (2) distinguishing kidney parenchyma, consisting of the cortex and me- Division of Nephrology, acute from chronic kidney diseases, and (3)detection University of Colorado dulla, is normally isoechoic (equal in brightness) or Denver, Denver of obstruction. hypoechoic (darker) compared with the normal liver Veterans Administration (2–4) or normal spleen. Thus, liver and spleen echo- Medical Center, 12700 East 19th Avenue, Box Use of Specific Parameters Obtained from genicity must be normal for comparison to be valid; fi C281, Aurora, CO Ultrasonography and Their Interpretation in particular, fatty in ltration of the liver can increase 80045. Email: sarah. Echoes and Echogenicity its echogenicity, making evaluation of the echogenic- [email protected] Proper interpretation of ultrasonography images ity of the right kidney more difficult. The cortex and depends on a basic understanding of how ultraso- medulla of the kidney generally have the same echo- nography images are generated. For diagnostic ultraso- genicity, although the medulla may be slightly darker nography, high-frequency sound waves are generated (5,6); both kidneys should have similar echogenicity and received by the ultrasonography transducer, which to each other. The renal sinus in the center of the is placed on the skin. Returning sound waves (echoes) kidney contains fibro-fatty tissue and thus appears are processed by a computer and displayed on a com- very bright relative to the kidney parenchyma. Bland puter screen. A gray-scale image is produced when the fluid (e.g., simple cysts), urine, and blood are an- ultrasonography machine operates in B-mode, or bright- echoic (black); therefore, the medullary pyramids, ness mode, in which returning echoes are represented which contain urine in parallel tubules, appear as as bright dots; the brightness of the dots represents the dark pools between the cortex and renal sinus. (See strength of the reflected echoes. Echogenicity, therefore, Figure 1 for the normal echogenic appearance of the refers to how bright or dark something appears in the capsule, parenchyma, medullary pyramids, and cen- gray-scale image; the brighter something appears, the tral renal sinus.) Blood clots in the urinary space will more echogenic it is. With regard to the kidney, echo- appear echogenic compared with the surrounding genicity generally refers to how bright or dark the kidney fluid; relative to the renal parenchyma, an acute clot parenchyma appears in comparison to the liver. is hypoechoic, while subacute and chronic clots are Normally, something that reflects most of the sound echogenic (brighter than renal parenchyma). waves back will appear the brightest (i.e., white); for What Is the Significance of Echogenicity in AKI example, fat and fibrous tissue are very echogenic and Other Kidney Diseases? Increased echogenicity and are therefore some of the brightest-appearing of the kidney parenchyma results from the increased 382 Copyright © 2014 by the American Society of Nephrology www.cjasn.org Vol 9 February, 2014 Clin J Am Soc Nephrol 9: 382–394, February, 2014 Use of Ultrasonography in Patients with AKI, Faubel et al. 383 Figure 1. | Grayscale ultrasonographic image demonstrating normal kidney size and echogenicity. (A) Grayscale longitudinal ultrasono- graphic image of a normal right kidney. The kidney is 11.2 cm long and surrounded by the bright rim of renal capsule (yellow arrow) around the parenchyma. The normal parenchyma (red line) forms a thick darker rim around the central bright (echogenic) sinus. Normal renal parenchyma is similar (isoechoic) or slightly darker (hypoechoic) compared with liver and includes the cortex and medulla. The thickness of the renal cortex is measured from the outer border of the medullary pyramids (yellow line) or from the arcuate arteries to the renal capsule. The medullary pyramids (white arrow) contain fluid in parallel tubules, which is anechoic (black) and appear as regularly spaced dark pools at the inner margin of the parenchyma (arrow). Because fat is echogenic, the central renal sinus appears bright. (B) Grayscale transverse ultrasonographic image of normal right kidney showing normal width (5.2 cm) and height (3.4 cm, anterior/posterior measurement) and appearance. (C) Grayscale longitudinal ultrasonographic image of normal right kidney next to a hyperechoic-appearing liver that has steatosis (because fat is echo- genic, the accumulation of fat increases the liver echogenicity/brightness). LNG, longitudinal view; MID, midline; RK, right kidney; TRV, transverse. presence of material that can reflect sound waves back, thus syndrome, complications of pregnancy (e.g., eclampsia), or increasing its brightness on the ultrasonography image. sepsis, which results in necrosis of tubular cells of the cortex; Because fibrous tissue (e.g., glomerulosclerosis, interstitial fi- the necrotic cells and resultant increase in interstitial fluid brosis) increases echogenicity, CKD is typically associated cause the decrease in cortical echogenicity (11). with increased echogenicity. Inflammatory infiltrates may Although clinically relevant kidney diseases may be present explain the increased echogenicity that occurs with acute in- without changes in echogenicity, if increased parenchymal terstitial nephritis and GN (7). Proteinaceous casts are echogenicity is noted (echogenicity is greater than a normal thought to cause the increased echogenicity associated liver) it is usually abnormal (except in neonates). For ex- with acute tubular necrosis (ATN) (8). Calcium deposits ample, increased echogenicity was reported to have a 96% and stones are very echogenic; thus, medullary nephrocalci- specificity (and 67% positive predictive value) for the presence nosis is characterized by increased medullary echogenicity of parenchymal kidney disease (3). and a relatively normal-appearing cortex (Figure 2). Several Use of Increased Echogenicity to Distinguish AKI from renal diseases are associated with medullary nephrocalcinosis, CKD. Increased echogenicity may occur in AKI or CKD and including medullary sponge kidney, hyperparathyroidism, re- cannot be used to reliably distinguish between the two. It is nal tubular acidosis, and vitamin D toxicity (9,10). Other kid- important to recognize, however, that in patients with CKD, ney diseases that have been associated with a hyperechoic it is unusual for kidney echogenicity to be less than that of the medulla include sickle cell disease and gout (9). liver. In one study, for example, only 11% of patients with Cortical necrosis is a rare cause of AKI that is classically CKD (serum creatinine, 8 mg/dl) had kidney echogenicity characterized by a dark, hypoechogenic renal cortex on ultra- that was less than liver echogenicity (2); of note, 30% of sonography. Cortical necrosis is a consequence of severe patients with AKI had kidney echogenicity that was less ischemia due to vascular abnormalities such as hemolytic-uremic than liver echogenicity. Furthermore, when ultrasonography 384 Clinical Journal of the American Society of Nephrology Figure 2. | Hyperechoic (bright) medulla from medullary nephrocalcinosis. Several kidney diseases